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In the third week of 2021, clinical labs nationwide tested 23,549 samples for influenza. Of those, only 0.3 percent (65 tests) tested positive – a number that is, to say the least, absolutely wild.
“Normally at this time of year we would be 20 to 30 percent positive,” said Lynnette Brammer, head of the home flu surveillance team at the Centers for Disease Control and Prevention.
Although the United States continues to fight COVID-19, it has apparently beaten the flu to submission. As of the end of September, the combined total of positive influenza cases identified by both public health labs and clinical labs has been below 1,500. There are high schools with more people. The phenomenon isn’t just in the United States – flu rates around the world are almost out of the ordinary. When you line up several years on the same graph, it may even appear that there are no flu cases this year. This is how out of step we are with the norm.
Flu cases fell in 2021
Number of specimens and those that tested positive for influenza, plus the proportion that tested positive, from CDC’s voluntary clinical and public health laboratory networks during the third week of January 2011-2021
| Year | Not tested | No. Pos. | Minute. Pos. |
|---|---|---|---|
| 2011 | 11,067 | 3,710 | 33.5% |
| 2012 | 5 364 | 320 | 6.0 |
| 2013 | 20,931 | 6,393 | 30.5 |
| 2014 | 17,480 | 4,680 | 26.8 |
| 2015 | 34 194 | 7 983 | 23.3 |
| 2016 | 20 761 | 1,658 | 8.0 |
| 2017 | 41,010 | 9,250 | 22.6 |
| 2018 | 75,417 | 22,642 | 30.0 |
| 2019 | 45 371 | 8,864 | 19.5 |
| 2020 | 65,248 | 18 967 | 29.1 |
| 2021 | 36,751 | 68 | 0.2 |
These data are mind-boggling, especially since many observers have spent the fall worrying about a “twindemic”. The tiny flu season also raises a few questions: How can there be so many cases of one respiratory illness and so few of another? Why isn’t there always a clear match between places with the least COVID-19 and places with the least influenza, or vice versa? And, seriously, are the flu numbers really so low?
The answers are complex, even if this tiny flu season is a simple fact.
We don’t follow the flu like we follow COVID-19. The average seasonal flu exists in a strange liminal space, severe enough to watch for, but not so severe that we literally try to count every case. After all, most people who get the flu won’t even bother to get tested. They will have a few bad days in bed (if they are able to quit work) and otherwise live life without change.
Instead, influenza case counts come from a few different surveillance systems, including a network of about 100 public health laboratories and 300 clinical laboratories that participate in virological surveillance, reporting weekly test numbers and CDC positive cases. There is also a network of doctors’ offices reporting ‘flu-like illness’ cases, a network of hospitals reporting laboratory-confirmed cases and mortality surveillance data from the National Center for Health Statistics, which pulls numbers. for influenza, pneumonia and other respiratory illnesses directly from death certificates. Other than NCHS numbers, none of these networks represent true national numbers. This is a sample from across the country, and they are reported voluntarily. This is why the annual number of influenza cases (and even deaths) is an estimate, extrapolated from the narrower table we can see.
All of this to say that there were almost certainly over 65 cases of the flu across the United States a few weeks ago. This number represents what was counted in member laboratories – not what existed in total. And even that number is likely an undercount, experts told me, because right now people with symptoms of respiratory illness are far more concerned about COVID-19 than the flu. “If people aren’t tested for it, we don’t find it,” said Janet Hamilton, executive director of the State Council and Territorial Epidemiologists. “And the protocol is not to test for influenza if you are negative for COVID.” There are likely many people whose fears of a COVID-19 infection are alleviated by a negative test result, and who simply go home and face the illness they make have without seeking any medical treatment.
Having said that, Hamilton and others have told me that there is reason to believe that actual influenza cases are indeed on the decline. Part of it is that we are seeing negligible flu seasons in other countries, which count their flu cases differently than we do. In addition, hospitalizations for influenza in the United States are also on the decline. “People in the hospital are correctly diagnosed. So that’s a better indication of lower blood flow in people, ”said Adolfo García-Sastre, director of the Influenza Pathogenesis Research Center at Icahn School of Medicine in New York City. Between October 1 and January 23, there were a total of 142 laboratory-confirmed influenza hospitalizations in this country – 0.5 per 100,000 Americans over almost four months. Typically at this time of year we are talking about tens per 100,000 every week.
This massive change, experts told me, is probably related to the precautions we took to avoid catching COVID-19: wearing a mask, social distancing, obsessive cleaning of surfaces (which doesn’t do much. to prevent COVID-19 but probably prevents the flu) and even prevents children from entering class. “Children are the main vector of influenza,” said David Topham, co-director of the New York Influenza Center of Excellence in Rochester. If they don’t breathe as usual, they can’t spread so much flu either. And this trick still works, even if the flu isn’t the reason we keep them at bay.
The flu has not been our target with all of these interventions, but we have certainly given it a blow. And that’s because the flu is just not as transmissible as COVID-19. The R0 (pronounced R zero) – the number that quantifies the average number of people who will catch a virus from a single infected person – is significantly lower for influenza than for COVID-19. “R nothing is generally 1 or 1.5 for the flu. And for SARS-CoV-2, it’s between 2 and 4, ”Topham said, referring to the scientific title of the new coronavirus. “When you do masking and social distancing, you can artificially lower R anything. This probably brings the flu down to a range below 1. Someone is infected, they infect less than another person, and then it is not viable.
Our strategies also work on COVID-19. But not so dramatically, as it was more likely to spread to more people initially.
That doesn’t fully explain why the places that have taken masking and distancing very seriously – Australia, for example – and the places that haven’t – say the United States – both have a season. influenza almost non-existent. Significantly reduced international travel likely played a role in that regard, Brammer said. Usually our influenza season follows that of the southern hemisphere. But if there weren’t a lot of them there and there wasn’t a lot of travel to carry the virus – the flu has no way to travel.
Meanwhile, in the United States, flu rates have remained low in both states that have instituted measures like mask warrants and those that have not. And García-Sastre said it was probably because of a little concept you may have heard of: collective immunity. Just as you don’t need to immunize absolutely everyone to gain an advantage in reducing disease transmission, you also probably don’t need everyone to wear masks and social distancing to. see a benefit.
Still, scientists aren’t sure what’s going on because the problem with a really, really tiny flu season is that it doesn’t leave you with enough cases to make solid statistical inferences. We don’t know, for example, what happens when you get both the flu and COVID-19 because there haven’t been enough cases to do a good research. We don’t really know how this bottleneck affects strains of flu that are circulating for the same reason. We don’t even know, for sure, that it’s the masks and distancing that are crushing the flu because there are so few flu cases left to review.
And it’s an irony that could end up affecting future flu seasons. Take the flu shot. Experts are using data from previous flu seasons to decide which strains people should be vaccinated against. “This will certainly make the selection of viruses for the vaccine for years to come more difficult. We just don’t have much to look at in the world, ”said Brammer. “What happens in the long run, I don’t know.”
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